test(rtx-fsi): RTX_FSI{2,3}_ES stiffness override for the TWIN-2 parameter sweep

The second knob of the TWIN-2 campaign (omni-cortex
docs/twin2_stiffness_campaign.md): case_from_env now also reads
RTX_{prefix}_ES over the case's benchmark Young's modulus, mirroring
UMEAN exactly — digit-identical with the knob unset (verified in vivo
on BOTH committed defaults against same-day baselines: FSI2 and FSI3
physics lines diff-clean, only compile/wall timings moved), a loud
override line naming the benchmark value and E/E0 when set. The
benchmark-inflow physics-band guard in both turek_hron tests widens to
a benchmark-CASE guard (u_mean AND e_s bit-identical) so no measured
band ever asserts off-benchmark; machinery invariants stay asserted at
every inflow and stiffness.

Co-Authored-By: Claude Fable 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01X2GmJXeQ2njUecEKiJZ1G2

crates/specialized/rtx-fsi/tests/fsi2_harness/mod.rs

@@ -80,12 +80,14 @@ pub const FSI3: BenchmarkCase = BenchmarkCase {
 };
 
 /// `RTX_{prefix}_UMEAN` over the case's benchmark inflow — TWIN-1's
-/// sweep parameter (`docs/twin_composition_campaign.md` in omni-cortex).
-/// Unset, the case comes back unchanged: the same f64 flows and the
+/// sweep parameter (`docs/twin_composition_campaign.md` in omni-cortex)
+/// — and `RTX_{prefix}_ES` over the case's benchmark Young's modulus —
+/// TWIN-2's second parameter (`docs/twin2_stiffness_campaign.md`).
+/// Unset, the case comes back unchanged: the same f64s flow and the
 /// march is digit-identical by construction (and verified in vivo on
 /// both committed defaults). When set, the march's printed rigid-drag
 /// reference still names the BENCHMARK value, which only applies at the
-/// case's own inflow — the override line below keeps logs honest.
+/// case's own inflow — the override lines below keep logs honest.
 pub fn case_from_env(prefix: &str, case: BenchmarkCase) -> BenchmarkCase {
     let u_mean = env_or(&format!("RTX_{prefix}_UMEAN"), case.u_mean);
     if u_mean.to_bits() != case.u_mean.to_bits() {
@@ -96,7 +98,20 @@ pub fn case_from_env(prefix: &str, case: BenchmarkCase) -> BenchmarkCase {
             100.0 * u_mean
         );
     }
-    BenchmarkCase { u_mean, ..case }
+    let e_s = env_or(&format!("RTX_{prefix}_ES"), case.e_s);
+    if e_s.to_bits() != case.e_s.to_bits() {
+        println!(
+            "  {} e_s OVERRIDDEN to {e_s:.4e} (benchmark {:.4e}; E/E0 = {:.4})",
+            case.name,
+            case.e_s,
+            e_s / case.e_s
+        );
+    }
+    BenchmarkCase {
+        u_mean,
+        e_s,
+        ..case
+    }
 }
 
 pub fn circle_sdf(x: f64, y: f64) -> f64 {